This invention relates to a method of producing interconnectors for solid oxide electrolyte fuel cells (SOFC).
In the manufacture of solid oxide electrolyte fuel cells, interconnector fabrication is one of the most important and troublesome processes. Interconnectors for solid oxide electrolyte fuel cells must have stable and sufficient electronic conductivity in a wide range of partial pressure of oxygen from the air electrode side to the fuel electrode side, show a coefficient of thermal expansion which is substantially equal to that of the electrolyte material (e.g. yttria-stabilized zirconia (YSZ)), and be unreactive with other cell constituent materials even at 1,273K. Owing to such severe requirements, alkaline earth metal-doped LaCrO.sub.x (lanthanum chromium (III) oxide) type oxides are generally used as interconnector materials. However, it is difficult to sinter these oxides and they will only sinter at temperatures at which other cell constituent materials will be damaged, namely 1,700K.
Therefore, the methods currently in use for interconnector fabrication include, among others, (1) the method comprising forming thin films using a dry process such as CVD (chemical vapor deposition)-EVD (electrochemical vapor deposition) or flame spraying, (2) the method comprising forming individual films using a hot press, and (3) the method comprising forming individual films by sintering or cosintering using a sintering aid. Among these methods, the method of fabricating interconnectors by forming thin films on electrode materials such as LaMnO.sub.3 (lanthanum manganate) type oxides or Ni/ZrO.sub.2 (nickel/zirconia) thermet by CVD-EVD or flame spraying is currently the most practical method. Thin films formed by this method have a thickness much smaller as compared with films formed by sintering and therefore are advantageous in that even when LaCrO.sub.x, which has a high electric resistance as compared with electrode materials, is used, the internal resistance of the cell can be reduced.
However, the CVD-EVD and flame spraying methods have the following disadvantages.
1. CVD-EVD method
a) Being essentially a CVD process, it needs a mask formed directly on (and in close contact with) a substrate for forming a thin film on a portion of the substrate. The masking and demasking steps thus become complicated. In addition, the masking material, which is used at high temperatures and in a chloride vapor atmosphere, is limited to certain species. PA1 b) Continuous processing is impossible. The productivity of batchwise processing is poor. PA1 c) Any dopants other than Mg (magnesium) cannot be used. PA1 a) Pinholes are readily formed. PA1 b) Selective vaporization of component materials occurs and possibly causes the composition of the thin films obtained to deviate from the desired composition. PA1 1) Since that portion of the target which is bombarded with laser beams is disintegrated and released without changing its composition, the film composition hardly deviates from the target composition. Therefore, thin films having a desired composition can readily be obtained by using a target having the same composition as the desired film composition. PA1 2) It is not necessary to bring a mask material into close contact with the substrate, masking and demasking are easy. PA1 3) Continuous production becomes possible by using an air lock system, leading to improved productivity. PA1 4) At a sufficiently high substrate temperature, ions and other particles diffuse on the substrate to some extent to give a stable crystal structure, so that pinhole-free compact thin films can be obtained.
2. Flame spraying method